Resin covers are widely used as display protectors for mobile electronic devices such as mobile phones and smartphones. Such resin covers, however, are exceeded by those made of glass in terms of excellence in transmittance, weather resistance, and damage resistance, and additionally, glass improves the aesthetics of displays. Accordingly, there has been an increasing demand for display protectors made of glass in recent years. Furthermore, a trend toward thinner and lighter mobile devices has naturally created a demand for thinner cover glasses. A cover glass is a component that has an exposed surface, and therefore is susceptible to cracking when exposed to an impact (e.g. contact with a hard object, dropping impact). Obviously, the thinner the cover glass is, the higher the probability of cracking is. Accordingly, a demand for a glass with sufficient mechanical strength is increasingly growing.
A possible strategy to solve the above problem is to improve the strength of cover glasses. The following two methods for strengthening glass plates have been known: thermal strengthening (physical strengthening); and chemical strengthening.
The former method (i.e. thermal strengthening) involves heating a glass plate nearly to its softening point and rapidly cooling the surface thereof with a cool blast or the like. Unfortunately, this thermal strengthening method, when performed on a thin glass plate, is less likely to establish a large temperature differential between the surface and the inside of the glass plate, and therefore less likely to provide a compressive stress layer at the glass plate surface. Thus, this method fails to provide desired high strength. Another fatal problem is that processing (e.g. cutting) of a thermally strengthened glass plate is difficult because the glass plate will shatter when a preliminary crack for cutting is formed on the surface. Additionally, as opposed to the above-mentioned demand for thinner cover glasses, the thermal strengthening method fails to provide desired high strength when performed on a thin glass plate because this method is less likely to establish a large temperature differential between the surface and the inside of the glass plate, and therefore less likely to provide a compressive stress layer at the glass plate surface. Accordingly, cover glasses strengthened by the latter method (i.e. chemical strengthening) are generally used instead.
The chemical strengthening method involves contacting a glass plate containing an alkali component (e.g. sodium ions) with a molten salt containing potassium ions to cause ion exchange between the sodium ions in the glass plate and the potassium ions in the molten salt, thereby forming a compressive stress layer for improving the mechanical strength at a surface layer of the glass plate. In the glass plate subjected to this method, potassium ions, which have a larger ionic radius than sodium ions, in the molten salt have replaced sodium ions in the glass plate, and thus are incorporated in a surface layer of the glass plate, which is accompanied by a volume expansion of the surface layer. Under the temperature conditions of this method, the glass cannot flow in a viscous manner at a speed high enough to relax the expansion. Consequently, the expansion remains as volume compressive residual stress in the surface layer of the glass plate, and improves the strength.
Surface compressive stress and depth of a compressive stress layer can be used as measures of the strength of chemically strengthened glasses.
The term “surface compressive stress” or simply “compressive stress” refers to compressive stress in the outermost layer of a glass plate, which is generated by incorporation of ions having a larger volume into a surface layer of the glass plate by ion exchange. A compressive stress cancels tensile stress that is a factor of breaking glass plates, and thus contributes to higher strength of chemically strengthened glass plates than that of other glass plates. Accordingly, the surface compressive stress can be used as a direct measure for the improvement of the strength of glass plates.
The “depth of a compressive stress layer” or simply “depth of layer” refers to the depth of a region where a compressive stress is present, as measured from the outermost surface of the glass plate as a standard. A deeper compressive stress layer corresponds to higher ability to prevent a large microcrack (crack) on the surface of the glass plate from growing, in other words, higher ability to maintain the strength against damage.
In addition to their thin but highly strengthened glass plate structures, another reason why chemically strengthened glass plates are commercially popular is that these glasses can be cut although they are already strengthened. In contrast, processing (e.g. cutting) of a glass plate already strengthened by the thermal strengthening method is difficult because the plate will shatter when a preliminary crack for cutting is formed on the surface.
It is generally known that thermally strengthened glass plates have a compressive stress layer having a depth of about ⅙ of the entire plate thickness at each surface of the glass. Strong tensile stress occurs in the inside glass region under this deep compressive stress layer to achieve a mechanical balance with the compressive stress in the compressive stress layer. If a preliminary crack for cutting the glass is formed to reach the tensile stress region, the tensile stress automatically propagates the crack to shatter the glass. This is why thermally strengthened glass plates cannot be cut.
On the other hand, a chemically strengthened glass plate is prepared by ion exchange in a micrometer-order thin superficial layer of the glass plate. Therefore, strictly speaking, the ion exchange depends on Fick's law of diffusion, but is often approximated by a linear function. As for chemically strengthened glass plates, their compressive stress layers and surface compressive stresses can be controlled by changing ion exchange conditions, and the compressive stress layers are very thin compared to those of thermally strengthened glass plates. Namely, the compressive stress layers and the surface compressive stresses of the chemically strengthened glasses can be controlled to avoid strong tensile stress that automatically propagates and leads to shatter of the glasses even when a preliminary crack for cutting is formed on the glass plate. This is why general chemically strengthened glasses can be cut.
Chemically strengthened glass plates can be cut as described above, but with great difficulty. Such cut difficulty causes breakage of glass plates to result in reduction of the yield of the resulting products. Therefore, chemical strengthening of preliminary cut glasses has been suggested (e.g. Patent Literature 1).
On the other hand, a trend toward lighter and thinner touch panels has naturally created a demand for chemically strengthened glass plates with higher strength. Therefore, for example, Patent Literatures 2 to 4 have suggested aluminosilicate glass as glasses suitable for chemical strengthening with a high ion exchange rate.
Further, in order to improve the cutting easiness of chemically strengthened glass plates, a method of relaxing a compressive stress of the outermost surface of a glass by a post treatment such as heating of the surface of a glass after chemical strengthening (e.g. Patent Literature 5) has been suggested.